The present invention relates to footwear, and more particularly to a sole construction for an article of footwear.
There is a continuing effort to provide ever more comfortable footwear. Running shoes, as well as other footwear, have undergone tremendous evolutionary advances in technology over the past 20 years. Many of the technological advances have occurred in the midsole. In most footwear, the midsole functions as the “suspension system” of the sole and it often provides both protective cushioning and a stable platform for the wearer's foot. Variations in the characteristics of the midsole can have a dramatic affect on the performance of the shoe. In an effort to provide improved performance, it is often desirable to vary the support characteristics of the sole from one region to another. For example, it may be desirable to provide a higher density material in the heel and a lower density material in the forefoot. A higher density material in the heel provides greater support upon heel strike while a lower density material provides appropriate cushioning and support for the typically smaller loads encountered in the forefoot. A wide variety of soles have been developed to provide variable support over the foot. In some applications, variable support is provided by forming different regions of the midsole from different materials, such as softer EVA foam in the forefoot and firmer EVA foam in the heel. In other applications, the sole is provided with a support plate that can be configured to provide the sole with the desired overall support profile. Although a marked improvement over conventional uniform sole constructions, there remains a need for a sole construction that is inexpensive to manufacture and that is highly tunable with a wide range of adjustability.
At the same time, there is also an ongoing effort to extend the life of footwear soles. In conventional footwear, the midsole (as well as other sole components) may begin to lose its performance over a relatively short period of time. Degradation of the sole material can cause the sole to lose its resiliency over time, particularly in regions of high and repeated impact, such as the heel. The rate of degradation will vary from sole to sole, but is largely dependent on the specific characteristics of the sole material and the types of loads applied to the sole. For example, conventional closed and open cell foams, such as EVA, have a relatively short life as the material naturally breaks down over relatively short periods of use. Conventional foam materials are also susceptible to temperature changes, which can cause the resiliency of the foam to vary noticeably. For example, the sole may become noticeably stiffer in colder temperatures and noticeably softer in higher temperatures. As a result, temperature can have a significant adverse affect on the support characteristics of a sole manufactured from conventional foam materials.
Accordingly, there remains a need for a highly reliable, highly tunable sole that has an extended life and is relatively inexpensive to manufacture.